Electrochemical batteries, such as fuel cell batteries or electrolytic cell batteries, often comprise a plurality of components stacked one on top of the other, e.g. electrolyte/electrode units, bipolar plates, cooling cards, pole plates, pressure pads, etc. To ensure a flow of current through said stack, the surfaces of the components have to make electrical contact with each other in the direction of the stack. The current should thereby pass through with as little loss as possible and therefore on the shortest route through the stack.
Uniform electrical contact between two adjacent components however poses problems, if the distance between these components is variable. This is the case for example with pressure pads, which are arranged in fuel cell batteries at a distance of several fuel cells within the cell stack. These pressure pads are supplied with a generally electrically insulating pressure medium and are at a variable distance from their boundaries.
Such a pressure pad can for example be structured by providing a hollow space between two adjacent cooling cards or a cooling card and a pole plate of a fuel cell stack, said space being supplied with a pressure medium. A specific distance is established between the two cooling card surfaces or the pole plate and cooling card surfaces as a function of the pressure supplied and the tolerances of the components.
To ensure the flow of current through the stack, these surfaces have to make electrical contact with each other, as the pressure medium is unable to do this. A component for making contact with the surfaces of said components must therefore have a variable geometrical extension in the direction of the stack and a large number of contacts per unit of surface. It is advantageous if it can cover a large productive route, as this means that the number of pressure pads in a fuel cell stack can be kept low.
EP 0 591 800 B1 discloses a fuel cell battery, in which a spring contact sheet is arranged for the transfer of current between an electrolyte/electrode unit and an adjacent cooling card or an adjacent pressure pad. The spring contact sheet has a flat sheet making electrical contact with an electrode of the electrolyte/electrode unit and spring tabs bent out of the plane of the sheet, the tips of which make electrical contact with the cooling card.
The spring tabs are hereby bent out sharply out of the plane of the sheet and form an acute angle with the cooling card. If said spring tabs are compressed between the two adjacent battery components, the tabs bend. This mechanical load causes very high bending stresses at the base points of the spring tabs in the plane of the sheet, i.e. in the region where the spring tabs leave the plane of the sheet. If a maximum permitted stress is exceeded, this point is deformed in a plastic and irreversible manner, so that the spring characteristics are impaired. The spring tabs or spring contact sheet can therefore only be compressed to a limited degree or the height to which the springs can be bent up, is restricted by the stress ratios.
The bending stresses also result in bending of the ridges between the tabs in the plane of the sheet. In order to prevent such bending, the ridges between the spring tabs have to be relatively wide and are for example approximately the length of the spring tabs. This restricts both the spring characteristics of the spring contact sheet and the number of spring tabs and thus the number of contacts per unit of surface.